In an optical transmission network via which a wavelength division multiplex (WDM) signal is transmitted, an optical signal-to-noise ratio (OSNR) is monitored and an optical device provided on an optical network is controlled such that the OSNR of a certain value or more may be achieved. Also, an error and a failure of an optical transmission path and an optical transmission apparatus may be detected by monitoring the OSNR. It is anticipated that, in next-generation optical networks, the wavelength and route of an optical signal will dynamically change and that there will be an increased demand for optical signal quality monitoring in the future.
As an OSNR monitoring method, a method in which received an optical signal is caused to branch out into two paths, an optical power is measured on one of the paths, a noise power Nbeat is measured on the other one of the paths, and an optical signal power Psig is obtained and an OSNR is calculated from Expression (1) has been known (for example, see U.S. Pat. No. 6,433,864).
                              N          beat                =                  2          ⁢                      A            R                    ⁢                                    P              sig              2                        ⁡                          (                                                1                  OSNR                                +                                                      B                    O                                                        2                    ⁢                                                                                  ⁢                                          R                      ·                                              OSNR                        2                                                                                                        )                                                          (        1        )            
In Expression (1), R is a measured resolution, B0 is a bandwidth of an optical signal, and A is a constant.